In situ determination of the syringyl/guaiacyl ratio of residual lignin in pre-bleached eucalypt kraft pulps by analytical pyrolysis

•A new method for S/G ratio analyses in lignin in pre-bleached pulp was developed.•S/G ratio in residual lignin in pre-bleached pulp can be measured by Py–GC/MS-SIM.•A pyrolysis based analysis of residual lignins in pulps has been developed.•S/G ratios of oxygen pre-bleached pulps were lower compared to unbleached pulps.•S/G ratios are lower for kraft pulps compared to wood sawdust.

Pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS) was used for evaluating the syringyl/guaiacyl (S/G) ratios of residual lignins in oxygen pre-bleached kraft pulps from 24 Eucalyptus wood clones. The optimization of the method showed that the best conditions were achieved when pyrolysis is carried out at 550 °C, using a 60 m length SPB-1701 capillary column and the mass spectrometer operating in SIM mode. Pulp pyrograms were analyzed and the compounds produced by pyrolysis of residual lignins identified by mass spectrometry. A total of 18 compounds were identified, being nine derived from guaiacyl units and nine derived from syringyl lignin. The S/G ratios were calculated using the relative area of S-lignin (syringol, 4-methylsyringol, 4-vinylsyringol and trans-4-(prop-2-enyl) syringol) and G-lignin (guaiacol, 4-methylguaiacol, 4-vinylguaiacol and trans-isoeugenol) as lignin markers. These studies showed that Py-GC/MS-SIM is an efficient tool to characterizing the chemical composition of residual lignin in oxygen pre-bleached Eucalyptus pulps.